Bistable liquid crystal device

ABSTRACT

This invention provides a bistable liquid crystal device. The bistable liquid crystal device includes a first substrate having thereon a first conductive layer and a first alignment layer, a second substrate having thereon a second conductive layer and a second alignment layer; and a liquid crystal layer sandwiched between the first and second alignment layers. The first alignment layer induces a first pretilt angle θ 1  in the range of 20°-65° between the liquid crystal layer in contact with the first alignment layer. The second alignment layer induces a second pretilt angle θ 2  in the range of 20°-65° between the liquid crystal layer in contact with the second alignment layer. The liquid crystal layer is capable of maintaining a stable bend state or a stable splay state at zero bias voltage and is switchable between the stable bend state and the stable splay state when a switching energy is applied in operation to the liquid crystal layer.

FIELD OF INVENTION

The present invention is related to a liquid crystal device, andparticularly to a bistable bend-splay liquid crystal device.

BACKGROUND OF INVENTION

Liquid crystal displays that are truly bistable under zero voltage biasare desirable for many practical applications. There are several typesof bistable displays based on liquid crystals. They all have theiradvantages and drawbacks. For example, there is a large body ofliterature on the bistable cholesteric display where the bistable statesare the focal conic and the planar alignment states. Another class ofbistable liquid crystal display is based on ferroelectric type liquidcrystals. Here the bistable states are both homogeneous alignment stateswith different orientations. The difference in orientations isdetermined by the angle of the dipole moment and the director of theliquid crystal molecules.

Yet another class of bistable liquid crystal display is based on thetwisted nematic effect in a liquid crystal display. It relies on theinterplay between the elasticity of the liquid crystal and the surfaceanchoring conditions. These are bistable twisted nematic displays wherethe bistable states are both twist states. In the Berreman bistabletwisted nematic liquid crystal display, the bistable twist states arezero twist and 360° twist states (See, D. W. Berreman and W. R. Heffner:J. Appl. Phys. 52 (1981) 3032; and D. W. Berreman: J. Opt. Soc. Am. 63(1973) 1374.). Kwok et al teaches a generalization of such bistabletwisted nematic displays where the bistable twist states are φ and φ+2πtwist states where φ can be several fixed values, both negative andpositive (See, H. S. Kwok: J. Appl. Phys. 80 (1996) 3687, T. Z. Qian, Z.L. Xie, H. S. Kwok and P. Sheng: Appl. Phys. Lett. 71 (1997) 596, Z. L.Xie and H. S. Kwok: Jpn. J: Appl. Phys. 37 (1998) 2572, and Z. L. Xieand H. S. Kwok: J. Appl. Phys. 84 (1998) 77.). These φ values have becalculated and experimentally verified. These Berreman bistable twistednematic displays can be called 2π-BTN displays (See, D. W. Berreman andW. R. Heffner: J. Appl. Phys. 52 (1981) 3032; and D. W. Berreman: J.Opt. Soc. Am. 63 (1973) 1374.).

Durand et al teaches another variant of the bistable twisted nematicdisplay where the bistable twist states are zero and 180° twist states(See, I. Dozov, M. Nobili and G. Durand: Appl. Phys. Lett. 70 (1997)1179.). The switching of such display is more difficult, but notimpossible, than the Berreman bistable liquid crystal displays. AgainKwok et al teaches a generalization of such π-BTN displays where thebistable twist states are φ and φ+π twist states, where φ can be one ofseveral published values (See, H. S. Kwok: J. Appl. Phys. 80 (1996)3687, T. Z. Qian, Z. L. Xie, H. S. Kwok and P. Sheng: Appl. Phys. Lett.71 (1997) 596, Z. L. Xie and H. S. Kwok: Jpn. J. Appl. Phys. 37 (1998)2572, and Z. L. Xie and H. S. Kwok: J. Appl. Phys. 84 (1998) 77.).

Yet there is another kind of bistable display, based on twisted nematicliquid crystals. It is an invention of Jones et al and is based on thebistable surface alignment conditions on an asymmetric grating surface(See, G. P. Bryan-Brown, C. V. Brown and J. C. Jones: Patent GB9521106.6 (October 1995).). The liquid crystal molecules just outsidethe grating surface can be either homogeneously aligned orhomeotropically aligned. This leads to a bistable alignment of theliquid crystal cell. This bistable display can be switched by theapplication of an electrical pulse to select either one of the surfaceconditions.

Further, Boyd et al (Appl. Phys. Lett. 36, 556 (1980)) presented abistable display based on the bend-splay deformation. That display wasalso based on a guest-host effect with absorbing dyes and thick liquidcrystal cells. The voltages needed for switching were very high andimpractical.

It is therefore the object of the present invention to provide anotherbistable liquid crystal device.

SUMMARY OF INVENTION

In accordance with the objects of the present invention, there isprovided in one aspect a bistable liquid crystal device having a firstsubstrate having thereon a first conductive layer and a first alignmentlayer; a second substrate having thereon a second conductive layer and asecond alignment layer; and a liquid crystal layer sandwiched betweenthe first and second alignment layers. The first alignment layer inducesa first pretilt angle θ₁ in the range of 20°-65° between the liquidcrystal layer in contact with the first alignment layer. The secondalignment layer induces a second pretilt angle θ₂ in the range of20°-65° between the liquid crystal layer in contact with the secondalignment layer. The liquid crystal layer is capable of maintaining astable bend state or a stable splay state at zero bias voltage and isswitchable between the stable bend state and the stable splay state whena switching energy is applied in operation to the liquid crystal layer.

In the preferred embodiment, the liquid crystal layer has a positivedielectric anisotropy and a cell gap-birefringence product 0.31±0.1 μm.

In another aspect of the present invention, there is provided a methodfor producing a bistable state in a bistable liquid crystal device. Thebistable liquid crystal device includes a first substrate having thereona first conductive layer and a first alignment layer, a second substratehaving thereon a second conductive layer and a second alignment layer,and a liquid crystal layer sandwiched between the first and secondalignment layers. The method comprises inducing a first pretilt angle θ₁in the range of 20°-65° between the liquid crystal layer in contact withthe first alignment layer; inducing a second pretilt angle θ₂ in therange of 20°-65° between the liquid crystal layer in contact with thesecond alignment layer; aligning the liquid crystal layer either in astable bend state or in a stable splay state at zero bias voltage; andapplying a switching energy to the liquid crystal layer to switch theliquid crystal layer between the stable bend state and the stable splaystate.

The bistable bend-splay liquid crystal device according to the presentinvention has better viewing angles, better contrast ratios, fasterselection and lower operating voltages than the other bistable displaysbased on the twisted nematic effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the angles of the liquid crystalmolecules relative to the alignment.

FIG. 2A is a perspective diagram illustrating how the liquid crystal isaligned in a typical bend state.

FIG. 2B is a perspective diagram illustrating how a liquid crystal isaligned in a typical splay state.

FIG. 3 is a schematic diagram showing the tilt angle of the liquidcrystal as a function of position inside the cell.

FIG. 4 is a cross-sectional view of a portion of a liquid crystal cellaccording to a preferred embodiment of the present invention.

FIG. 5 is an SEM photograph of the surface of 150 nm SiO_(x) layerdeposited at an angle of 85°.

FIG. 6 is a perspective diagram illustrating a preferred embodiment ofthe structure of a passive matrix display.

FIG. 7 is cross-sectional view of a portion of a liquid crystal cellhaving patterned electrodes according to another preferred embodiment ofthe present invention.

FIG. 8 is a perspective diagram illustrating an interdigital structureof conductive layer electrodes.

FIG. 9 is a schematic diagram showing driving waveform of the electricalpulses to the interdigital electrodes.

FIG. 10 is a perspective diagram illustrating a structure of the passivematrix display with interdigital electrodes.

FIG. 11 is experimental demonstration results showing transmission ofthe cell as a function of the electrical pulses.

FIG. 12 is a diagram showing experimentally measured transmissionspectra of the BBS display.

FIG. 13 is a diagram showing driving voltages needed as a function ofthe duration of the driving pulses.

FIG. 14 is a diagram showing driving voltages needed as a function ofthe duration of the driving pulses.

DETAILED DESCRIPTION

Referring to FIG. 1, the alignment of the liquid crystal molecules in aliquid crystal cell may be described by the director orientation n(z) inthe one-dimensional approximation. The vector function n(z) is fullyrepresented by the functions θ(z) and φ(z), where the θ(z) is the polarangle and φ(z) is the azimuthal angle of the vector n(z). The alignmentof the liquid crystal is governed by minimization of the total elasticenergy which leads to the usual Euler-Lagrange equations as given byformulas (1) and (2) $\begin{matrix}{{{2k_{1}\overset{¨}{\theta}} + {\frac{\mathbb{d}k_{1}}{\mathbb{d}\theta}{\overset{.}{\theta}}^{2}} - {\frac{\mathbb{d}k_{2}}{\mathbb{d}\theta}{\overset{.}{\phi}}^{2}} - {\frac{\mathbb{d}k_{3}}{\mathbb{d}\theta}\overset{.}{\phi}} - {D^{2}\frac{\mathbb{d}}{\mathbb{d}\theta}\left( \frac{1}{ɛ_{zz}(\theta)} \right)}} = 0} & (1) \\{{{2k_{2}\overset{.}{\phi}} + k_{3}} = {constant}} & (2)\end{matrix}$where k₁, k₂ k₃ and ∈₌ are given byk ₁(θ)=K ₁₁ cos² θ+K ₃₃ sin² θ  (3)k ₂(θ)=(K ₂₂ cos² θ+K ₃₃ sin² θ)sin² θ  (4)k ₃(θ)=2q _(o) K ₂₂ cos² θ  (5)∈_(zz)(θ)=θ_(//)−Δ∈ cos² θ(z)  (6)and D is the electric displacement. In equations (3) to (6), K₁, K₂₂ K₃₃are, respectively, the splay, twist and bend elastic constants of theliquid crystal mixture, and ∈_(//) and ∈_(⊥) are the anisotropicdielectric constants and Δ∈=∈_(//)−∈_(⊥) is the dielectric anisotropy.The equilibrium alignment configuration of the liquid crystal cell canbe obtained by solving equations (1) and (2), subjected to the boundaryconditions, which are determined by the surface treatment of the liquidcrystal cell.

The solution of nonlinear coupled differential equations (1) and (2) isoften not unique. There can be multiple solutions. Since the solutionsare obtained by the minimization of the total elastic energy E, one hasto calculate the elastic energy for the various stable configurations inorder to find the solution with the absolute minimum energy. If thevarious stable solutions have nearly the same energy, multiple solutionswill exist, which is the basis of bistability. Take the simplest case ofa liquid crystal cell with a boundary conditions of φ(0)=φ(d)=0; andθ(0)=θ₁ and θ(d)=−θ₁, where θ₁ is a constant and d is the cell gap. IfK₃₃ and K₁₁ are nearly the same, it can be shown that the following twosolutions will both satisfy the boundary conditions and equation (1):$\begin{matrix}{{{\theta(z)} = {\theta_{1}\left( {1 - \frac{2z}{d}} \right)}}{and}} & (7) \\{{\theta(z)} = {\theta_{1} + {\left( {\pi - {2\theta_{1}}} \right)\frac{z}{d}}}} & (8)\end{matrix}$

The reason is that the boundary conditions θ₁ and π−θ₁ are equivalent.Now equation (7) represents a splay cell and equation (8) represents abend cell respectively, as shown in FIGS. 2A and 2B. In particular,under a bend state, liquid crystal molecules 2 will align themselvesvertically at and near the mid-point between substrates 1 as shown inFIG. 2A, and under a splay state, liquid crystal molecules 3 will alignthemselves horizontally at and near the mid-point between the substrates1 as shown in FIG. 2B.

The dependences of θ(z) for the two cases are also graphically shown inFIG. 3. Each of these two solutions represents a local minimum in thetotal elastic energy space.

It is possible to find out the condition where the two solutions areequally likely by equating the elastic energy of the splay and benddeformation cells. The total elastic energy per unit wall area is givenby the equation $\begin{matrix}{E = {\frac{1}{2}{\int_{0}^{d}{\left( {{K_{11}\sin^{2}\theta} + {K_{33}\cos^{2}\theta}} \right)\theta^{2}{\mathbb{d}z}}}}} & (9)\end{matrix}$

If the splay and bend cells have the same elastic energy, and if K₃₃ andK₁₁ are nearly the same, then the following equation can be derived:(K ₃₃ −K ₁₁)sin 2θ₁+(π−4θ₁)(K ₃₃ +K ₁₁)=0  (10)

By solving this equation, the condition for the pretilt angle such thatthe splay and bend deformation energies are the same can be obtained.For example, for p-methyoxybenzylidene-p′-butylaniline (MBBA),K₃₃/K₁₁=1.3. Hence θ₁ is about 47°. In general it can be shown that θ₁is always between 45° and 58° for all values of K₃₃/K₁₁. Under thecondition that equation (10) is satisfied, bistability can be obtained.Actually bistability can be achieved even if the deformation energiesfor the bend and splay cells are slightly different.

There is actually another possible solution to equation (1). It is aretwist cell. It can be proved that this retwist state has a much highertotal elastic energy than both bend and splay state and can be ignored.

Thus, given the proper boundary condition and pretilt angle, the stablealignment configuration of the liquid crystal cell can either be a splayor a bend cell. This is the basis of our invention. Our inventioninvolves the design of such a bistable bend-splay display and methods ofdriving it. In the practical device, the pretilt angles on the liquidcrystal cell surfaces can be different. Thus it is possible to haveθ(0)=θ₁ and θ(d)=−θ₂. For the general case, the bend and splay elasticdeformation energies are the same, and thus bistability can be achievedif the following condition is satisfied: $\begin{matrix}{{{\frac{1}{2}\quad\left( {K_{33} - \quad K_{11}} \right)\quad\begin{pmatrix}{{\sin 2\theta}_{1} +} \\{\quad{\sin\quad 2\theta_{2}}}\end{pmatrix}} + \quad{\left( {K_{11} + \quad K_{33}} \right)\quad\left( {\pi - \quad{2\quad\theta_{1}} - \quad{2\quad\theta_{2}}} \right)}} = \quad 0} & (11)\end{matrix}$This equation is a generalization of equation (10). An example of θ₁ andθ₂ that can satisfy this equation when K₃₃/K₁₁=1.3 is θ₁=30° and θ₂=65°.

As well, there can be a small degree of twist in the stable alignment inorder to improve the optical properties. Thus it is possible that φ(0)=0and φ(d)=φ_(o) for some values of φ_(o).

Our invention is based on bistable bend and splay alignment of theliquid crystal cell. It can be called the bistable bend-splay liquidcrystal display, or BBS display in abbreviation.

Referring now to FIG. 4, the liquid crystal cell includes a firstsubstrate 4A and a second substrate 4B, typically made from two piecesof glass. The inner surfaces of said first and second substrates 4A and4B are respectively coated with conductive layers 5A and 5B, typicallywith transparent conductive layers of indium tin oxide (ITO). The ITO isthen patterned to various shapes according to the display to be desired,which will be described in detail later. Typically, liquid crystalalignment layers 6A and 6B are respectively deposited onto saidconductive layers 5A and 5B. In addition, a liquid crystal layer 7 issandwiched between said first and second alignment layers 6A and 6B.Further, a pair of input and output polarizers (not shown) arepositioned in parallel over said first and second substrates 4A and 4B.Typically, said pair of input and output polarizers respectively makesan angle of ±40° to ±60° with the liquid crystal alignment direction.Said first alignment layer 6A induces a first pretilt angle θ₁ betweensaid liquid crystal layer 7 in contact with said first alignment layer6A, and said second alignment layer 6B induces a second pretilt angle θ₂between said liquid crystal layer 7 in contact with said secondalignment layer 6B. In particular, the alignment layers 6A and 6B aretreated such that the pretilt angle of liquid crystal molecules 8A nearsaid first alignment layer 6A make an angle of θ₁ to the surface of saidfirst alignment layer 6A, and liquid crystal molecules 8B near saidsecond alignment layer 6B make an angle of θ₂ to the surface of saidsecond alignment layer 6B respectively. The liquid crystal molecules inthe middle of the liquid crystal cell will be at an angle that dependson the driving conditions. To maintain the bistable bend-splay states atzero bias voltage, large pretilt angles θ₁ and θ₂ are required.Typically, θ₁ and θ₂ in the range of 20°-65° are required. Said pretiltangles θ₁ and θ₂ can be the same value or substantially different.

The large pretilt angle that is required for bend-splay bistability canbe obtained in one of several ways. The simplest way is to usephotoalignment. It has been reported in the literature thatphotoalignment can be used to produce pretilt angles from 0 to 90° byadjusting the irradiation conditions. So it is possible to make the40°-50° needed for θ₁ and θ₂. It is also possible to use normal rubbingof polyimide to make large pretilt angles. The special polyimiderequired is the so-called side-chain polymers.

Another method to make strong anchoring at large pretilt angles is bySiO_(x) evaporation. We have experimental demonstration of the BBSdisplay using SiO_(x) evaporation, even though it is not the only way todo so. In our experimental demonstration, the glass plates with ITOelectrodes are treated by oblique SiO_(x) evaporation with evaporationangle of 85°, and thickness of 60˜150 nm these being known to give theoblique anchoring of the liquid crystal molecules. Different thicknessgives different anchoring energy. The pretilt angle of LC using isaround 45° measured by the traditional crystal rotation method.

In the evaporation system the distance between source and sample shouldbe larger so that the uniformity can be obtained over the entire regionof the sample. FIG. 5 shows the scanning electron micrograph photographof the surface of 150 nm SiO_(x) layer deposited at an angle of 85°.This surface can produce the 45° pretilt of the liquid crystalmolecules.

Yet another method of obtaining a high pre-tilt angle is by using amixture of vertical and horizontal alignment material to produce thealignment layer. In particular, following are two examples for obtaininghigh pre-tilt angles.

EXAMPLE 1

Example 1 shows the procedures adopted to prepare an alignment layercapable of providing a pretilt angle of 44 degree.

Materials:

-   -   The horizontal alignment material: purchased from Japan        Synthetic Rubber Company (model number: JALS9203), which was in        a solution form. (JSR Corporation, 5-6-10 Tsukiji Chuo-ku,        Tokyo, 104-8410, Japan.) The solvent in JALS9203 comprises        γ-butyrolactone (γBL), methyl-2-pyrrolidone (NW), and Butyl        cellosolve (BC).    -   The vertical alignment material: purchased from Japan Synthetic        Rubber Company (model number: JALS2021), which was in a solution        form. The solvent in JALS2021 comprises methyl-2-pyrrolidone        (NWP) and Butyl cellosolve (BC).    -   Substrate: an ITO glass coated with electrodes, purchased from        Nanbo Company, Shenzhen, China.        Procedures:

0.95 g of horizontal alignment material solution and 0.05 g of thevertical alignment material solution were mixed together and stirredthoroughly. The mixture was applied to the substrate to obtain a softsolid film using spin coating. The spin coating was first operated at800 rmp for 10 sec and then at 3500 rmp for 100 sec. A soft filmconsisting of the horizontal and vertical alignment materials was formedwith remnant solvents.

In order to drive out all the remnant solvents and to cure the polymers,the coated glass was placed in an oven. It was first baked at 100° C.for 10 min (soft bake) and then baked at 230° C. for 90 min (hard bake).A hard film, i.e., the alignment layer, was formed.

The surface of the alignment layer was subjected to rubbing treatmentusing a nylon cloth in such a way that the layer was rubbed in onedirection one time.

Result:

The pretilt angle of the alignment layer produced accordingly to Example1 was 44 degree.

EXAMPLE 2

Example 2 shows the procedures adopted to prepare an alignment layercapable of providing a pretilt angle of 53 degree.

Materials:

-   -   The horizontal alignment material: purchased from Japan        Synthetic Rubber Company (model number: JALS9203), which was in        a solution form.    -   The vertical alignment material: purchased from Japan Synthetic        Rubber Company (model number: JALS2021), which was in a solution        form.    -   Substrate: an ITO glass coated with electrodes, purchased from        Nanbo Company, Shenzhen, China.        Procedures:

0.5 g of the solution of horizontal alignment material and 0.5 g of thesolution of the vertical alignment material were mixed together andstirred thoroughly. The mixture was applied to the substrate to obtain asoft solid film using print coating as follows:

A stainless steel rod of 2 cm diameter and 5 inches long was placed onthe substrate. A few drops of the mixture were placed underneath the roduntil it spread out along the contact line between the rod and thesurface of the substrate. The rod was then rolled or slided along thesubstrate surface to form a liquid film.

The coated substrate was then placed on a hot plate at 100° C. for 10min to drive out all the solvents. It was then put in an oven for hardbaking at 230° C. for 90 min. A hard film consisting of vertical andhorizontal alignment materials was then obtained. The spin coating wasfirst operated at 800 rmp for 10 sec and then at 3500 rmp for 100 sec. Asoft film consisting of the horizontal and vertical alignment materialswas formed with remnant solvents.

In order to drive out all the remnant solvents and to cure the polymers,the coated substrate was placed in an oven. It was first baked at 100°C. for 10 min (soft bake) and then baked at 230° C. for 90 min (hardbake), forming a hard film, i.e., the alignment layer. The surface ofthe alignment layer was subjected to rubbing treatment using a nyloncloth in such a way that the layer was rubbed in one direction one time.

Result:

The pretilt angle of the alignment layer produced accordingly to Example2 was 53 degree.

In example 1, the liquid film becomes a soft solid film by spin coating.The solvent is evaporated slowly so that the domains of H and V tend tobe larger. Also the ratio of surface areas of H and V domains will favorthe material that has a higher solubility in the mixed solvent since thematerial with a lower solubility will precipitate first.

In example 2, the solidification is fast due to heating on a hot plate.Thus the domains tend to be smaller. The area ratio of the H and Vdomains will not be affected too much by the different solubility of thematerials.

The pretilt angles obtained by the procedures in example 1 and example 2are different, even for the same mixture of H and V alignment agents.This is because of the different domain structures obtained using thedifferent procedures.

When a switching energy is applied in operation to the liquid crystallayer 7, the liquid crystal therein can be switched between the stablebend state and the stable splay state. Typically, the switching energyis an electrical pulse generated by the first and second conductivelayers 5A and 5B. In one embodiment, the first and second conductivelayers 5A and 5B are patterned into stripes that are substantiallyperpendicular in direction to each other to form an overlapping matrixof pixels. In particular, for a matrix display, the ITO is patternedinto horizontal and vertical stripes 9 and 10 with small gap between thestripes. The intersection area between the top and bottom electrodesrespectively of the first and second conductive layers 5A and 5B formsthe pixel area as shown in FIG. 6. In another embodiment, both the firstand second conductive layers 5A and 5B are transparent. Voltages areapplied to the top and bottom electrodes to switch the liquid crystalcell to either the bend state or the splay state of alignment. Forexample, the electrical pulse having low frequency can align the liquidcrystal layer 7 to the bend state, and the electrical pulse having highfrequency can align the liquid crystal layer 7 to the splay state.Alternately, a dual frequency driving method can be used to drive theBBS with the help of a dielectric anisotropy that changes sign as thedriving frequency is changed. For the bend state, we need a positive Asso that when a voltage is applied between the top and bottom electrodes,the mid-plane liquid crystal molecules will align themselves vertically,thus favoring the bend alignment. As the driving frequency is change sothat Δ∈ is negative, then the mid-plane molecules will favor ahorizontal alignment. Thus a splay deformation is obtained.

Now both the splay and bend states are so-called birefringence modes fora liquid crystal display. The transmission of these states is given bythe equationT=cos²(α−γ)−sin 2α sin 2γ sin² δ  (12)where α and γ are the polarizer and analyzer angles; δ is the phaseretardation of LC cell which is given by $\begin{matrix}{\delta = {\frac{\pi}{\lambda}{\int_{0}^{d}{\left( {{n_{e}(\theta)} - n_{o}} \right){\mathbb{d}z}}}}} & (13)\end{matrix}$where n_(e)(θ) is the extraordinary refractive index, no is the ordinaryrefractive index of the liquid crystal material, and d is the thicknessof the liquid crystal cell. As usual Δn=n_(e)−n_(o) is the birefringenceof the liquid crystal material. α, γ and dΔn can be optimized to obtainthe best optical properties. Our goal is to achieve the highest contrastratio (CR) which is the brightness of the on-state divided by theresidual brightness of the dark state, and less dispersion for brightstate. The optimization shows that α=−γ=45°, and dΔn=0.30˜0.32 μm. Sincethe display mode is the same as the normal birefringence mode liquidcrystal display, therefore odd multiples of this value 0.30˜0.32 μm canalso be used to make the liquid crystal display, at the expense ofhigher color dispersion. Thus it is possible to have dΔn=0.93 μm, 1.55μm as well.

Generally, for common LC materials, large n_(e)-n_(o) results in largerΔ∈ which implies a faster response. But larger n_(e)−n_(o) means thatthe cell gap should be smaller for a fixed dΔn. The cell gap cannot betoo small due to manufacturing yield. We have made three kinds of LCcells. The details are shown in Table 1. In particular, the LC materialsin Table 1 can be obtained from Merck Company. All of them showed thebend-splay bistabilities. The 3.25 μm cell gap case is interestingbecause it has a large Δ∈ and an appropriate cell gap. But it should bestressed that any cell gap can be used in our bistable bend-splaydisplay. TABLE 1 LC mode selection d (μm) LC materials n_(e) − n_(o) Δε1.5 5CB or 5700-100 0.15˜0.18 25.8 3.25 5700-000 or 7500-000 0.1˜0.1223.6 or 10.2 5 88Y1104 0.07 10.4

In a second preferred embodiment of the BBS, the driving of the cellfrom the bend configuration to the splay configuration is achievedthrough a horizontal electric field. The basic cell structure of thisBBS is shown in FIG. 7. It is essentially the same as FIG. 4 except thatITO electrodes 11 and 12 of the first and second conductive layers 5Aand 5B are now patterned. In particular, the bottom electrode 12 has aninterdigital structure as shown in FIG. 8. In the experimentaldemonstration, the interdigital electrodes are 4 μm wide and spaced 6 μmapart, resulting in a pitch of 10 μm. Other dimensions are possible.

For this preferred embodiment, the driving method is shown in FIG. 9.The top electrode 11 is biased at a common voltage V_(c). Electricalpulses are applied to the interdigital electrodes 11 and 12. Theopposing digits are given voltages of V₁ and V₂ respectively. This isdifferent from the common in-plane switching method of liquid crystaldisplays in which the top electrode is floated. But this comparison isirrelevant to our invention anyway. The important point here is that wecan control the voltages on the interdigital electrodes 11 and 12 suchthat either a vertical or horizontal electric field is imposed upon theliquid crystal molecules.

In the simplest case, V_(c) is kept constant at ground. Two electricalpulse trains V₁ and V₂ are applied to the bottom electrodes. When V₁ andV₂ are the same, either positive or negative, the electric field insidethe liquid crystal cell is in the vertical direction. The liquid crystalalignment will favor the bend state. When V₁ is opposite in sign to V₂,the electric field inside the liquid crystal cell is horizontal(in-plane). Thus the splay state is obtained. The efficiency ofswitching is dependent of the spacing of interdigital electrodes, theliquid crystal cell gap and the amplitude and duration of the electricalV₁ and V₂ pulses.

The interdigital electrode is amenable to a matrix arrangement formaking a passive matrix display. The arrangement of the top and bottomelectrodes is depicted in FIG. 10. The common top electrodes are used asscan electrodes while data is fed to the interdigital lines. In thiscase, V_(c) also participates in the driving scheme. Thus it is possibleto multiplex the driving scheme to make a high-resolution display withno cross talk.

In order to test the effectiveness of this preferred embodiment, wefabricated three kinds of liquid crystal cells as shown in Table 1. Allof these cells can show BBS bistability and can be switched. Here wecite the results of the 3.25 μm cell as an example. In this example, theglass plates with ITO electrodes are treated by oblique SiO_(x)evaporation with evaporation angle of 85°, and thickness of 150 nm.Under this condition, the pretilt angle was 45°. The rectangular pulsesof voltage U and duration τ are applied. The experimental results areshown in FIG. 11. Here, U=27V and τ=1 ms. We used the same voltage forboth bend state and the splay state. However, the voltages needed forthe bend state can be different from that of the splay state. Forcircuitry considerations in a matrix-driving scheme, having the samevoltage is a little more convenient to control.

FIG. 12 shows the transmission spectra of the splay and the bend states.It can be seen that the wavelength dispersion for this display is quitesmall. The dark state is quite dark, giving an experimentally measuredCR of 45. It is possible to improve the contrast and the lighttransmission efficiency by adding a half-wave plate between thepolarizers. From theoretical simulations, CR of over 200 can be achievedwith white light illumination.

Generally, the switching voltage is related to the duration of theswitching pulse. For a short pulse, a higher voltage is required. Allexperimental data obtained in this preferred embodiment were obtainedusing SiO_(x) evaporation with a 45° pretilt angle. The method to obtainthe high pre-tilt angle alignment here is conventional. For example, T.Uchida et al teach such method (see, T. Uchida, M. Ohgawara and M. Wada:Japn. J. Appl. Phys. 19, 2127 (1980)). FIGS. 13 and 14 show ourexperimental data. Here we varied the duration of the driving pulses V₁and V₂, and measure the voltage needed for switching. It can be seenthat the smallest duration that can achieve bend-splay switching is 50μs. However, over 85V is needed. For a 1 ms pulse, the voltage needed is28V. From FIG. 12, it can be seen that the voltage needed for switchingfor this 3.25 μm cell is less than 10V for a 10 ms pulse. This is wellsuited for a matrix display using conventional driver electronics.

In a third preferred embodiment of the present invention (not shown infigures), the liquid crystal cell is designed as a one-polarizerreflective display. In particular, one of the interdigital electrodes orthe common electrodes is optically reflecting. The configuration of thisliquid crystal cell is the same as the transmittive display, except thathere, either the interdigital electrodes or the common electrode is nowmade of a reflective metal. The polarizer on the side of the metallicelectrode also can be eliminated since it will now serve no function. Asin the previous cases, the LCD cell with a high pre-tilt layer wasachieved by SiO_(x) evaporation.

The reflectivity of this liquid crystal display is given byT=1−sin²2α sin²2δ  (14)

Again, optimal optical performance is possible for α=45°. However, forthe reflective display, since light traverses the liquid crystal celltwice, the birefringence cell gap product dΔn should be half of that ofthe transmittive case. For this reflective cell, the LC modes are listedin Table 2. TABLE 2 LC mode selection d (μm) LC materials n_(e) − n_(o)Δε 0.8 5CB or 5700-100 0.15˜0.18 25.8 1.63 5700-000 or 7500-000 0.1˜0.1223.6 or 10.2 2.5 88Y1104 0.07 10.4

Other values are possible, with the help of a retardation film tocompensate for better optical properties. Also, for the sake ofmanufacturing and mass production, a larger cell gap may be needed atthe expense of compromised optical performance.

In a further embodiment of the present invention, the bistable displaycan be driven in a passive matrix mode. With the finger electrodes V₁and V₂, and the common voltage V_(c) properly designed, a passive matrixdisplay can be designed with no crosstalk, regardless of the multiplexnumber. This is a major advantage over non-bistable displays where thereis crosstalk associated with multiplexing. An example of the values ofV₁, V₂, and V_(c) can be shown in Table 3 for the case of 1 ms pulses.TABLE 3 Switching behavior of a passive matrix bistable display V₁(volt) V₂ (volt) V_(c) (volt) Final state 15 −15 0 Splay 28 28 0 Bend 15−15 15 No change 28 28 15 No change

The common rows are used as the addressing electrodes. Select voltage is0 and nonselect voltage is 15V. The data lines are V₁ and V₂. Here acombination of V₁ and V₂ gives the data signal. A combination of 15V and−15V gives the splay state while a combination of 28V and 28V give s thebend state. All of these switching behaviors have been verifiedexperimentally.

All the voltages can be offset by a constant voltage without affectingthe behavior of the display. For example, 15V can be subtracted from allthe voltages to make the select voltage −15V and the nonselect voltage 0for the scan lines as follows: TABLE 4 Switching behavior of a passivematrix bistable display V₁ (volt) V₂ (volt) V_(c) (volt) Final state 0−30 −15 Splay 13 13 −15 Bend 0 −30 0 No change 13 13 0 No change

Other voltages are possible as long as switching can be achieved withoutcrosstalk. For example, for the case of a 10 ms pulse, the followingcombinations should provide proper multiplexing of the passive matrixdisplay: TABLE 5 Switching behavior of a passive matrix bistable displayV₁ (volt) V₂ (volt) V_(c) (volt) Final state 10 −10 0 Splay 10 10 0 Bend10 −10 10 No change 10 10 10 No change

If we subtract 10V from all the electrodes, there will be no change inthe behavior of the display, and the following table can be obtained:TABLE 6 Switching behavior of a passive matrix bistable display V₁(volt) V₂ (volt) V_(c) (volt) Final state 0 −20 −10 Splay 0 0 −10 Bend 0−20 0 No change 0 0 0 No change

This represents a very simple driving scheme, as V₁ is kept constant atall times. V₂ is switched in a manner similar to ordinary multiplexingof STN liquid crystal displays. Thus one can use commercial STN LCDdrivers to drive this bistable display.

The present invention has been described in detail herein in accordancewith certain preferred embodiments thereof. To fully and clearlydescribe the details of the invention, certain descriptive names weregiven to the various components. It should be understood by thoseskilled in the art that these descriptive terms were given as a way ofeasily identifying the components in the description, and do notnecessarily limit the invention to the particular description. Forexample, the substrates are typically made from glass, but may be madeof any material that can interact properly with the alignment layer, theelectrodes and the liquid crystal cell. Further, a few methods ofobtaining the high pre-tilt angle are provided, but the presentinvention is not limited to the provided methods. The present inventionis applicable as long as the high pre-tilt angle can be achieved.Therefore, many such modifications are possible. Accordingly, it isintended by the appended claims to cover all such modifications andchanges as falling within the true spirit and scope of the presentinvention.

1) A bistable liquid crystal device comprising: a first substrate havingthereon a first conductive layer and a first alignment layer; a secondsubstrate having thereon a second conductive layer and a secondalignment layer; and a liquid crystal layer sandwiched between saidfirst and second alignment layers, said first alignment layer inducing afirst pretilt angle θ₁ in the range of 20°-65° between said liquidcrystal layer in contact with said first alignment layer, and saidsecond alignment layer inducing a second pretilt angle θ₂ in the rangeof 20°-65° between said liquid crystal layer in contact with said secondalignment layer, said liquid crystal layer being capable of maintaininga stable bend state or a stable splay state at zero bias voltage andbeing switchable between said stable bend state and said stable splaystate when a switching energy is applied in operation to said liquidcrystal layer. 2) The device of claim 1, wherein said liquid crystallayer comprises liquid crystal having a positive dielectricbirefringence when driven by electrical pulses at low frequency and anegative birefringence when driven by electrical pulses at highfrequency. 3) The device of claim 1, wherein at least one of said firstand second alignment layers comprises a mixture of vertical alignmentmaterial and horizontal alignment material. 4) The device of claim 1further comprising input and output polarizers. 5) The device of claim 4wherein said input and output polarizers respectively angle saidalignment direction by ±40° to ±60°. 6) The device of claim 1 whereinsaid pretilt angles on said pair of substrates are substantiallydifferent. 7) The device of claim 1 wherein said pair of substrates havesubstantially parallel alignment directions. 8) The device of claim 1wherein said switching energy is an electrical pulse generated by saidfirst and second conductive layers. 9) The device of claim 1 whereinsaid switching energy is an electrical pulse having low frequency toalign said liquid crystal layer to said bend state. 10) The device ofclaim 1 wherein said switching energy is an electrical pulse having highfrequency to align said liquid crystal layer to said splay state. 11)The device of claim 1 wherein said switching energy is an electricalpulse providing an electrical field in a predetermined direction betweensaid pair of substrates to switch said liquid crystal layer between saidbend state and said splay state. 12) The device of claim 1 wherein oneof said conductive layers further includes a patterned electrode toprovide an electrical field in a predetermined direction between saidpair of substrates to switch said liquid crystal layer between said bendstate and said splay state. 13) The device of claim 1 wherein one ofsaid conductive layers further includes a patterned electrode, saidpatterned electrode having an interdigital structure so that controllingthe voltages on said interdigital electrode can apply either a verticalor horizontal electric field upon said liquid crystal layer. 14) Thedevice of claim 1 wherein said first and second conductive layers arepatterned into stripes that are substantially perpendicular in directionto each other to form an overlapping matrix of pixels. 15) The device ofclaim 1 wherein both said first and second conductive layers aretransparent. 16) The device of claim 1 wherein one of said first andsecond conductive layer is optically reflecting. 17) In a bistableliquid crystal device, said bistable liquid crystal device including afirst substrate having thereon a first conductive layer and a firstalignment layer, a second substrate having thereon a second conductivelayer and a second alignment layer, and a liquid crystal layersandwiched between said first and second alignment layers, a method forproducing a bistable state comprising: inducing a first pretilt angle θ₁in the range of 20°-65° between said liquid crystal layer in contactwith said first alignment layer; inducing a second pretilt angle ƒ₂ inthe range of 20°-65° between said liquid crystal layer in contact withsaid second alignment layer; aligning said liquid crystal layer eitherin a stable bend state or in a stable splay state at zero bias voltage;and applying a switching energy to said liquid crystal layer to switchsaid liquid crystal layer between said stable bend state and said stablesplay state. 18) The method of claim 17 wherein applying said switchingenergy further comprises generating an electrical pulse by said firstand second conductive layers. 19) The method of claim 17 whereinapplying said switching energy further comprises applying a lowfrequency electrical pulse to align said liquid crystal layer to saidbend state. 20) The method of claim 17 wherein applying said switchingenergy further comprises applying a high frequency electrical pulse toalign said liquid crystal layer to said splay state. 21) The method ofclaim 17 wherein applying said switching energy further comprisesgenerating an electrical field in a predetermined direction between saidpair of substrates to switch said liquid crystal layer between said bendstate and said splay state. 22) A bistable liquid crystal devicecomprising: a first substrate having thereon a first conductive layerand a first alignment layer; a second substrate having thereon a secondconductive layer and a second alignment layer; and a liquid crystallayer sandwiched between said first and second alignment layers, saidliquid crystal layer having a positive dielectric anisotropy under a lowfrequency electrical field and a negative dielectric anisotropy under ahigh frequency electrical field, said first alignment layer inducing afirst pretilt angle θ₁ in the range of 20°-65° between said liquidcrystal layer in contact with said first alignment layer, and saidsecond alignment layer inducing a second pretilt angle θ₂ in the rangeof 20°-65° between said liquid crystal layer in contact with said secondalignment layer, said liquid crystal layer being either in a stable bendstate or in a stable splay state at zero bias voltage; and beingswitchable between said stable bend state and said stable splay statewhen a switching energy is applied in operation to said liquid crystallayer. 23) A bistable liquid crystal device comprising: a firstsubstrate having thereon a first conductive layer and a first alignmentlayer; a second substrate having thereon a second conductive layer and asecond alignment layer; and a liquid crystal layer sandwiched betweensaid first and second alignment layers, said liquid crystal layer havinga positive dielectric anisotropy and a cell gap-birefringence product of0.31±0.1 μm, said first alignment layer inducing a first pretilt angleθ₁ in the range of 20°-65° between said liquid crystal layer in contactwith said first alignment layer, and said second alignment layerinducing a second pretilt angle θ₂ in the range of 20°-65° between saidliquid crystal layer in contact with said second alignment layer, saidliquid crystal layer being either in a stable bend state or in a stablesplay state at zero bias voltage; and being switchable between saidstable bend state and said stable splay state when a switching energy isapplied in operation to said liquid crystal layer.